There has been a striking rise in obesity over the last two decades, together with parallel increases in incidences of insulin resistance, type-2 diabetes and cardiovascular disease. A critical link between obesity and downstream cardiometabolic disorders is chronic, low-grade inflammation, promoted by secretion of pro- inflammatory mediators by adipocytes and macrophages in visceral adipose tissue (VAT). While several positive effectors of adipose-tissue inflammation have been identified and characterized, negative regulators remain poorly defined. In 2009, we reported a population of Foxp3+CD4+ regulatory T (Treg) cells - with a distinct gene-expression profile and T cell receptor repertoire that is highly enriched in the VAT, but not lymphoid tissue, of lean mice as they age. This population is strikingly and specifically reduced in several insulin-resistant mouse models of obesity. Loss- and gain-of-function experiments demonstrated that Tregs can indeed regulate adipose tissue inflammation, as well as local and systemic metabolic indices. VAT Tregs have proven to be a paradigm for regulatory T cell populations specifically adapted to survive and function at particular tissue locales. Over the last funding cycle, we made substantial strides in elucidating the biology of VAT Tregs: identifying their defining transcription factor, PPAR?; extending the evidence for their role in controlling metabolic indices; delineating factors that drive their accumulation in aging lean mice, notably Ag/MHCII and the IL-1 family member, IL-33; dissecting their response to obesity; and transposing the tissue Treg concept to skeletal muscle. Building on these findings, the overall goal of this proposed project is to elucidate the molecular pathways that control VAT Treg dynamics. Three Specific Aims will undertaken, each designed to address a hypothesis that emerged from results obtained during the last funding cycle. The Aims are connected by their potential to explicate the generation of specialized tissular Treg populations. We propose to: 1) Elucidate how IL-33 drives accumulation of Tregs in VAT. 2) Determine whether and how TNF-a drives the loss of VAT Tregs in obese mice. 3) Determine how PPAR? and Foxp3 collaborate to impose the VAT Treg phenotype. These studies should yield important new information on molecular pathways involved in regulating the devastating downstream consequences of obesity. Identification of novel therapeutic targets within these pathways is especially important given unacceptable side-effects of the front-line thiazolidinedione family of type-2 diabetes drugs.